The present invention pertains to convertible fixed wing aircraft of the type that can be flown as a more-or-less conventional fixed wing aircraft, or converted to an automotive vehicle. The idea of a single vehicle that can function as an aircraft to cover long distances in the shortest time, and then as a highway vehicle that can be driven from the airport to the ultimate destination, is an attractive one. However, there are many problems that make this ideal concept difficult to realize, and while many have attempted to design such a vehicle, there has never been a successful product that has reached the market.
The problem has been in integrating the functions and structural requirements of the two vastly different vehicles into a single, user-friendly construction that requires a minimum of human intervention in switching from the automotive to the aircraft configuration. The technological bases for both individual systems are highly developed, and it is necessary that this high level of technology for both systems be incorporated in a single system while maintaining the performance of these different systems.
The structural design requirements for an aircraft are quite different from those of an automobile, particularly in the matter of weight, which must be held to the minimum level, whereas in an automobile, weight is not nearly as important a consideration, although the present tendency is toward lighter weight automobiles for fuel efficiency reasons. On the other hand, safety requirements for automobiles make energy absorption zones in the form of front and rear bumpers an absolute necessity, whereas in an airplane there is no need for such devices, as the airplane operates in open space, far removed from pedestrians and other traffic.
In automobiles, the standard body configuration is rectangular in planform, with four wheels positioned adjacent the four corners, and the center of gravity located more or less centrally between the wheels for maximum road stability. In aircraft, on the other hand, the fuselage is elongated and usually provided with tricycle landing gear, with one front wheel in the nose of the aircraft, and two main wheels located closely adjacent the center of gravity. This arrangement is necessary to provide a short moment arm between the center of gravity and the main wheels, while the elongated fuselage provides a long moment arm between the main wheels and the elevators, so that a relatively small downward force by the elevators is able to overcome the moment created by the weight of the aircraft at the center of gravity, to tilt the nose of the airplane up to the high angle-of-attack attitude for liftoff.
Another distinctive difference between aircraft and automobiles is that airplanes have wings that extend laterally for a considerable distance to each side of the fuselage, and a tail structure including horizontal and vertical stabilizers, that extends rearwardly for a considerable distance behind the main wheels, whereas in automobiles the body must be compact, usually no more than six feet in width, and at most not more than eight feet, to meet highway legal requirements, with a minimum overhang behind the rear wheels.
Aerodynamic requirements of an airplane are much higher than those of an automobile, primarily because of the different speeds at which the two operate. The aerodynamic drag of an automobile body is many times that of an airplane fuselage, due in part to the unstreamlined underbody, with all of the exposed power transmission and suspension gear.
Steering controls for aircraft are quite different from those for automobiles, in that turns are made by use of pedal-controlled rudders and wheel-controlled ailerons, whereas in an automobile turns are made by merely turning the front wheels with the steering wheel. The automobile steering wheel must be turned almost two full turns to the left or right to obtain maximum turning of the front wheels, while in aircraft the ailerons are deflected to the maximum position with only approximately ninety degrees turn of the wheel to either side. One of the primary problems in designing a convertible airplane is integrating the steering controls for both automotive and aircraft use, using only one control wheel, without requiring the operator to manipulate some kind of device in switching from one mode of control to the other. The airplane's elevators are also operated by pulling and pushing the control wheel.
Braking systems for aircraft and automobiles are also entirely different from one another. In the airplane, brakes on the main wheels are separately and independently controlled by tilting the left or right rudder pedals so as to brake the left or right main wheel. In this way, the airplane is made to turn on the ground. To brake the airplane without turning, both rudder pedals are tilted forwardly the same amount. Combining the rudder and brake controls in the same pedals is essential during the landing operation, so that the pilot is enabled to control both rudder and brakes. With the automobile, depressing the brake pedal actuates all four brakes simultaneously. The problem is how to fulfill all of these functions without confusing the pilot or driver who is used to both of these arrangements.
Another difference in the control systems for the two different types of vehicles is that in aircraft, the engine speed is controlled by a manually operated throttle lever, whereas in the automobile the engine speed is controlled by a foot-operated accelerator pedal.
Still another difference is that in aircraft the fuel is carried in the wing, relatively close to the center of gravity, therefore as the fuel is used up there is very little shift in the location of the center of gravity. With an automobile, on the other hand, fuel is carried in tanks at the rear portion of the body for safety reasons, and as the fuel is used up there can be a more substantial shift in the center of gravity. Airplanes can be trimmed for the relatively minor shift in the center of gravity by trimming the elevator or horizontal stabilizer, with its relatively long moment arm, using a trim wheel.